The short story opens with a brief paragraph from the narrator explaining that Roger Button and his wife have decided their first child should be born in a hospital, though at the time this was unusual as most babies were born at home. The year is 1860 and the Buttons live in Baltimore where Roger is the financially successful owner of Roger Button & Co., Wholesale Hardware. The Buttons are prominent members of the elite in antebellum Baltimore society. 

Roger hopes to have a boy that he can enroll at Yale University one day, where Roger himself graduated college. On the September morning when the baby is being born, Roger dresses fastidiously and rushes to the Maryland Private Hospital for Ladies and Gentlemen, with a mixture of nerves and excitement. Just outside the hospital, Roger happens to see Doctor Keene, the family physician. Roger notices a strange expression on Doctor Keene’s face and worriedly asks about his wife and newborn baby. Doctor Keene does not answer directly, and he seems to be annoyed by something to do with the birth of Roger’s child. Roger presses the doctor for details about what happened but the doctor snaps that Roger should go and see for himself. Doctor Keene angrily complains that his reputation could be ruined and that he never wants to see Roger Button or his family again. Doctor Keene then abruptly leaves.

Roger Button, now terrified, walks hesitatingly through the hospital doors. He approaches the nurse at the desk and identifies himself. The nurse’s cheery demeanor vanishes instantly, and she appears ready to run away. Roger asks to see his child and the nurse tells him to go upstairs. Roger goes to the second floor where another nurse greets him. He again identifies himself and asks to see his child. The nurse is so startled that she drops the basin she is holding and it clanks down the stairs. She agrees to take Roger to see the child but complains that the hospital’s reputation has been ruined.

Roger follows the nurse to the room where newborns are kept. He asks which baby is his and the nurse points. What Roger sees confuses him. Stuffed into one of the cribs and wrapped in a white blanket is a man who appears to be seventy years of age. Roger Button balks at the suggestion that this is his newborn baby, but the nurse assures him that it is. The old man asks Roger whether he is his father and if Roger can take him out of there. Roger Button at first refuses to believe that this old man is his newborn baby, but the nurse again assures him it is and insists that Roger must take him home today. The old man expresses relief at this, complaining about the crying of all the other babies and that they only give him milk to consume.

Roger Button stands stupefied and begins to imagine the embarrassment at having to bring home this “child.” The nurse snaps that Roger and his child need to leave immediately, and the old man announces that he won’t leave the hospital wearing only a blanket. The nurse then commands Roger to go into town and buy the old man some clothes. As Roger leaves, the old man calls out to him and says not to forget to buy him a cane as well.

At the store, there is some confusion between Mr. Button and the clerk. Mr. Button doesn’t think it proper to dress his son as a man, but of course boy’s clothes won’t fit him. Not wanting to admit the truth of his unheard-of predicament, Mr. Button winds up purchasing a fancy suit for an adolescent. When he presents the suit to the old man back at the hospital, the old man is hesitant. He doesn’t want to look foolish. Mr. Button insists that he put it on saying that it is the old man who has made him look foolish. The old man dons the suit, and the result depresses Mr. Button. It is a suit for a young boy, with dotted socks and pink pants, and it looks completely ridiculous on the old man. To try and make up for this, Mr. Button chops off the old man’s long grey beard with hospital shears. Gathering his courage, Mr. Button takes his son’s hand and leads him out of the hospital. When his son asks him what he is to be called, Roger jokes dryly that they’ll call him “Methuselah.”

Mr. Button persists in trying to make his son, now named Benjamin, appear as “normal” as possible. He has Benjamin’s hair dyed dark and his beard shaved close. He has a confused tailor make clothing for a young boy but specially fitted for a five-foot-eight adult. Despite these efforts, Benjamin still looks like an old man and walks with a stooped back. Mr. Button continues unwaveringly. At first, he forces Benjamin to eat milk instead of food, then relents and allows him bread and butter as well. He brings home toys for babies and children and commands Benjamin to play with them. However, Benjamin’s “old man” tastes cannot be suppressed. He smokes his father’s cigars and reads the encyclopedia.

At first, the news of the Buttons’ new child is a scandal in Baltimore. But when the Civil War breaks out the following year, the Buttons’ drama is forgotten. Benjamin continues to do his best to please his parents. He patiently endures play dates with adolescent boys in the neighborhood. When Benjamin accidentally breaks a window with a slingshot, his father is almost proud of him. Thereafter, Benjamin tries to break something every day. However, Benjamin finds that he is most comfortable in the company of his grandfather, talking about daily events.

At age five, Benjamin is sent to kindergarten, but he is bored by it and frequently dozes off during activities. The teacher complains to his parents and Benjamin is removed from the school, to his great relief. By age twelve, Benjamin’s parents begin to get used to him. Around this time, Benjamin looks in the mirror and discovers that he seems to have gotten younger these past twelve years. In any event, he no longer stoops and feels better physically than ever before. Benjamin works up the courage to ask his father to be able to wear long trousers, like a grown man should. His father, who has promised himself to treat Benjamin as any other twelve-year-old boy, resists at first. After some discussion, the two compromise and Benjamin is allowed to wear long trousers so long as he continues to dye his hair and play with boys his own age.

As years pass, Benjamin Button grows younger. Eighteen-year-old Benjamin, who looks about fifty now, takes the Yale exams and enrolls as a freshman. When he shows up at the registrar’s office, the registrar mistakes him for his father. Benjamin tries to convince the registrar that he is eighteen-year-old college freshman Benjamin Button, but to no avail. The registrar throws Benjamin out of the office, calling him a dangerous lunatic. On his way back to the train station, Benjamin notices that a crowd of students has gathered and is following him. The people in the crowd jeer at him and make fun of him as he boards the train. As the train pulls away, Benjamin yells out the window that they will all regret this.

Benjamin Button begins to work for his father’s company. Now that he’s twenty years old, his father insists on taking him out socially, to dances and parties. At one such party, Benjamin meets and instantly falls in love with Hildegarde Moncrief, the daughter of a prominent man. Hildegarde agrees to a dance with Benjamin. She mistakes Benjamin for his father’s brother, and Benjamin, remembering what happened at Yale, does not correct her. Hildegarde confesses that she likes older men and the two get along well. Benjamin spends the rest of the night in a love-induced daze. 

Six months later, Benjamin and Hildegarde make their engagement known. This causes a flurry of rumors to swirl about Benjamin and his true identity. None of these stories tell the real truth and Benjamin becomes known as the Mystery Man of Maryland. Roger Button publishes Benjamin’s birth certificate as proof of his authenticity, but people refuse to believe it because of Benjamin’s appearance. Because of all the fake stories, Hildegarde won’t even believe the real one, that Benjamin is a man of twenty. 

Over the course of the next fifteen years, the Buttons’ hardware business grows and flourishes, mostly due to Benjamin’s business savvy. The family becomes so successful that high society in Baltimore has no choice but to welcome them, despite past rumors. Benjamin himself feels and looks younger than ever. His energy levels are high and he relishes having fun. He even becomes the first citizen of Baltimore to own an automobile. His father now adores him, and Benjamin even shores up his relationship with his father-in-law. Unfortunately, as Benjamin grows younger he becomes less and less attracted to Hildegarde. She is getting older and slowing down while he is getting younger and speeding up. Benjamin becomes so restless that he decides to enlist in the army when the Spanish-American War begins. Benjamin serves admirably and is awarded a medal and met by a brass band when he returns.

Upon returning home from the war, Benjamin looks at himself in the mirror and is surprised to discover he looks younger than ever. He had hoped that his backward aging would stop once his true age matched his appearance, but that does not appear to be happening. He remarks about this to Hildegarde, which annoys her. She angrily accuses him of doing it on purpose and refuses to believe Benjamin can’t help his condition. Feeling more energetic than ever, Benjamin continues to drag Hildegarde out to parties. He dances while she watches disapprovingly. Others remark how sad it is for Benjamin to be tied to a wife so much older than him. Benjamin takes up golfing and becomes an avid dancer. As he and his wife continue to grow apart, Benjamin begins to contemplate handing over the family business to his now-grown son, Roscoe.

In 1910, Benjamin enrolls as a freshman at Harvard. He does well immediately, joins the football team, and becomes a star player in the game against Yale. However, each subsequent year, Benjamin becomes less adept at football, as he gets younger and slighter. By the time he is a senior, Benjamin looks like a freshman and does not make the team at all. Classes become too hard for him.

After graduation, Benjamin moves back to Baltimore. Hildegarde is now living in Italy, so Benjamin moves in with Roscoe. Cohabitation is difficult. Roscoe is a prominent member of society and does not want any drama or scandal. Benjamin is mostly in the way as he grows younger still. When Benjamin asks Roscoe to enroll him at a prep school for boys, Roscoe refuses. Roscoe reprimands Benjamin for continuing to grow younger and demands that he call him “Uncle” instead of Roscoe, to make things seem more normal.

A depressed Benjamin begins thinking about the ongoing war in Europe, which America has just joined. In fact, just then there’s a knock on the door, and the butler hands Benjamin a letter. It is from the U.S. Army explaining that soldiers who fought in the Spanish-American War are being called back into service. The letter commissions Benjamin as brigadier-general and orders him to report for duty immediately. Benjamin excitedly rushes to the tailor to be fitted for a uniform and later boards a train for Camp Mosby, South Carolina to train. When Benjamin arrives at camp, the sentry guard refuses to believe he is who he says he is. Benjamin raises a fuss, but no one believes him. Two days later, his son Roscoe shows up to take him home.

By 1920, Benjamin has the appearance of a ten-year-old boy, and Roscoe has his first child. Roscoe is embarrassed that his son’s grandfather looks so young. He refuses to believe anything other than that Benjamin is doing this on purpose and he denounces it as perverse and inefficient. When Roscoe’s boy is five, he and Benjamin look the same age and are taken care of by the same nurse. They both go to kindergarten, and this time around, Benjamin loves it. After a year, Roscoe’s son moves on, but Benjamin stays in kindergarten. After three years, Benjamin is deemed too young to be in kindergarten and goes home to be looked after by the nurse full time. Benjamin lives the life of a toddler, repeating “new” words, jumping on the bed, and playing soldier. Benjamin no longer ruminates or puzzles over things. He does not remember his adulthood. He does not dream. Soon, all he is aware of is when he is hungry, and he no longer understands what other people are saying when they speak. In the end, everything about him fades from his mind and all is dark.